46 [Assembly 



structure is plainly shown in one of the cases of the Hamilton 

 group, where a Phacops hufo is placed under a magnifier which 

 distinctly exhibits the eye and its lenses. 



The middle portion or back of the trilobite is "covered by a 

 series of jointed armor-like scales or ribs, extending from side to 

 side, and so articulated as to allow of great flexibility; many 

 species having had the power to roll themselves up into a globu- 

 lar form, in which condition they are often found fossil. These 

 ribs or articulations vary in number from six to fifteen or even 

 more. 



The posterior extremity is covered by a single plate, some- 

 times small, at other times nearly equaling in size that covering 

 the head. It is in some species nearly smooth, but is more gene- 

 rally marked with transverse furrows like those separating the 

 ribs of the body. It sometimes terminates in one or more sharp 

 spines ; and such indeed in a few species are found at the corners 

 of the bucklers, at the ends of the articulations, and even over 

 the whole surface of the shelly covering. 



All the Trilobites are marked by two furrows running from 

 front to back, dividing them apparently into three lobes, whence 

 they have their general name. The whole family have been 

 extinct since the Carboniferous period, and [horn that time fev/ 

 forms are found bearing any resemblance to them. One of the 

 living Crustacea most analogous to them is the Limulus or 

 "Horse-foot;" a specimen of which lies in the case with the trilo- 

 bites of the Trenton limestone to illustrate their structure. 

 They appear to have possessed but very slight and perishable 

 feet or paddles ; as among all the thousands of specimens which 

 have been found, no trace of these organs has been discovered. 

 The readiness with which the ligaments connecting their difier- 

 ent plates decayed, allowing these fragments to fall asunder 

 before being fairly buried in the sediment, explains the fact that 

 we find so few of them in a perfect and unmutilated form ; and 

 it is not improbable that many of the pieces we find belonged to 

 cast shells, which may have been annually shed by the growing 

 trilobites, as the coverings of the lobster and limulus are now 

 thrown off and renewed every year. 



The most abundant species in the Trenton limestone are the 

 Isotelus gigas, a remarkably large form ; and the Calymene senaria, 

 a small one, fragments of which are seen in almost every block 

 at Trenton Falls. Another very peculiar species is the Trinu- 



